1. Technical Field
The present disclosure relates to controlling liquid dispensers. In specific embodiments, the present disclosure relates to the selection and application of drive waveforms to piezoelectrically actuated drop-on-demand liquid dispensers so as to aspirate and dispense in a known and controlled fashion picoliter range droplets of a liquid (for example, an ink or a liquid containing chemically or biologically active substances).
2. Description of Related Art
Piezoelectrically actuated microdispensers and print heads are used to generate microdrops of various fluids in a wide range of non-contact microdispensing applications, such as ink jet printing, biological microarrays, miniaturized chemical assays, drug dosing, synthetic tissue engineering, rapid prototyping, security printing, micro-manufacturing of optic and electronic components, and precision application of lubricants and other specialty or high value liquids.
These microdispensers and print heads, like drop-on-demand piezo dispensers and ink jet print head devices, include a transducer or transducer array that is typically driven by a pulsed rectilinear or polygonal waveform control signal to cause fluid ejection through a small orifice. Due to complex interactions between the materials and electromechanical structure of the microdispenser, physical and Theological properties of the fluid, applied fluid pressure, and the applied drive waveform, many modes of stable or unstable fluid ejection are possible, such as drops, sprays, or elongated slugs of fluid.
The physical construction of the microdispenser or print head typically is fixed in microdispensing and ink jet printing systems, however fluid properties can vary according to the requirements of the end user's application. In many applications it is necessary or desirable to provide fluid drops, either mono-size or multi-size, having selectable drop volume and drop velocity that are ejected either satellite-free or in a manner such that satellite drops merge relatively quickly with the main drops.
One typical drop-on-demand piezo dispenser comprises a borosilicate glass capillary tube that is heat drawn and cleaved at one end to form an ejection orifice (orifices in the range 30-70 μm are common). A tubular piezoelectric transducer is bonded onto the capillary tube over a second heat drawn fluid restrictor element in the capillary tube. Piezo dispensers of this type are available from a number of sources including PerkinElmer Life & Analytical Sciences (formerly Packard Instrument Company of Downers Grove, Ill. or Packard BioScience of Meriden, Conn.). All piezoelectrically actuated drop-on-demand microdispensing and ink jet devices operate in accordance with the same fundamental squeezing principle: the piezoelectric transducer changes the volume of a fluid chamber within the device in response to an applied voltage pulse to eject a fluid droplet through a small orifice.
Reference is now made to FIG. 1 wherein there is shown a block diagram for a conventional system 10 for producing droplets of a fluid. The system 10 includes at least one piezoelectric drop-on-demand (DOD) dispenser 12 which is actuated in response to an electrical control signal 14 (also referred to as the drive signal) generated by a piezoelectric driver 16. The dispenser 12 may have one of several piezoelectric actuation configurations including, for example, a cylindrical squeezer-type capillary tube piezo dispenser (a microdispenser) for use in dispensing a liquid containing chemically or biologically active substances or an ink jet piezo printing head for use in dispensing a printing ink or specialty fluid. The piezo driver 16 includes a high voltage amplifier capable of generating voltage signals with levels up to about ±150 volts. The piezo driver 16 outputs the control (drive) signal 14 in response to an input signal 20 received from a rectilinear or polygonal pulse generator 18. The pulse generator 18 is configured to synthesize a particular waveform as the input signal 20 having certain known characteristics (height, width, rise time, fall time, delay time, and the like). The input signal 20 waveform is then amplified by the piezo driver 16 for application to the dispenser 12 as the control (drive) signal 14. The piezoelectric transducer within the dispenser 12 responds to the applied control (drive) signal 14 and ejects fluid (generally in the form of one or more droplets) from the orifice.
Oftentimes it is not possible to model or otherwise predetermine drop ejection characteristics with a high degree of predictive accuracy for a particular drive signal waveform with a particular fluid in a particular type of piezo dispenser, microdispenser or print head. Modeling of satellite drop formation and merging behavior is especially difficult to perform and is frequently deficient in predicting these physical phenomena correctly. As interactions between the piezo dispenser, microdispenser or print head, fluid, applied fluid pressure, and applied drive waveform are inherently complex, drive waveforms were principally discovered and developed using empirical methods.
The piezoelectric transducer of a drop-on-demand dispenser (for example, an ink jet device) is typically driven by either a rectilinear or polygonal voltage pulse shape drive signal waveform having a selected one of a variety of unipolar or bipolar and single or multiple pulse configurations. Generally, the shape of the drive signal waveform is related to deformation of the fluid cavity, motion of the fluid meniscus in the ejection passage, drop ejection through the orifice, and subsequent motion of the fluid meniscus. Such rectilinear or polygonal drive signal waveforms have also been used successfully in piezo dispensers (microdispensers) including PerkinElmer Piezo Tips for ejecting a liquid containing chemically or biologically active substances.
FIGS. 2-5 illustrate examples of known rectilinear or polygonal drive pulse shapes for the signal 20 generated by the pulse generator 18 for use in actuating a drop-on-demand piezoelectric dispenser 12 in the system 10 of FIG. 1. The rectangular drive pulse illustrated in FIG. 2 has been used to drive a standard PerkinElmer 70 μm Piezo Tip (the dispenser 12) so as to eject a single droplet having a volume of about 330 picoliters with a speed of about 2 m/sec. The illustrated rectangular drive pulse may have a pulse width of about 30 μsec, and when amplified by the piezo driver 16 to generate the control signal 14 may have a pulse height of about 65 Volts. FIG. 3 illustrates a double-pulse waveform which is taught by U.S. Pat. No. 5,736,994 for driving a piezoelectric shear mode-shared wall ink jet print head. It is known in the art to use such a waveform to drive a conventional drop-on-demand piezo dispenser in a configuration like that illustrated in FIG. 1 so as to eject single droplets using certain combinations of pulse parameters (for example, height, width, rise time, fall time, delay time). FIG. 4 illustrates a bipolar double-pulse waveform which is taught by U.S. Pat. No. 5,124,716. It is known in the art to use this waveform to drive a laminated piezoelectric bender-type ink jet printhead in a configuration like that illustrated in FIG. 1 so as to eject single droplets using certain combinations of pulse parameters (for example, height, width, rise time, fall time, delay time). Lastly, FIG. 5 illustrates a bipolar multi-segment pulse waveform which is taught by U.S. Pat. No. 6,513,894 for use in a configuration like that illustrated in FIG. 1 for the stable ejection by a piezo dispenser of droplets that are smaller than the diameter of the ejection orifice.
Microarraying applications are intrinsically diverse due to several differentiating factors, such as array size, spot density, sample types, buffer solutions, and substrate types, plus capacity and throughput requirements. For example, array sizes vary tremendously, ranging from about 100 to 50,000+ elements. Spot spacing typically decreases as array size increases, and thus a commensurately smaller drop volume is required in order to prevent spot overlapping on the substrate. It is recognized by those skilled in the art that rectilinear or polygonal drive signal-based piezo dispenser systems largely cannot, with respect to the diverse and special needs of microarraying applications, provide a broad range of fluid drop sizes having selectable drop volume and drop velocity, and further that are ejected either satellite-free or in such a manner that satellite drops merge relatively quickly with a main drop.
It is further recognized in the ink jet printing and fluid dispensing art that smaller drop volumes are preferred in some instances. Rectilinear or polygonal drive signal-based piezo ink jet dispenser systems appear to have a low limit drop size which is primarily dependent on orifice size. However, as orifice size decreases in ink jet applications, and thus smaller drops are potentially generated, the danger of clogging increases due to particulates that are carried by the ink (or that are present in the surrounding environment, such as air borne particulates) being dispensed through the smaller orifice. It is therefore desirable to keep the orifice size as large as possible while simultaneously satisfying requirements for smaller drop volumes.